1. Field of the Invention
This invention relates to tensioning of seabed-to-vessel marine risers. More particularly, this invention relates to tensioning the marine risers with a plurality of tensioning units in combination with a decoupled buoyancy can platform.
2. Description of the Related Art
A problem presented by offshore hydrocarbon drilling and producing operations conducted from a floating platform is the need to establish a sealed fluid pathway between each borehole or well at the ocean floor and the deck of the platform at the ocean surface. A riser typically provides this sealed fluid pathway. In drilling operations, the drill string extends through a drilling riser serving to protect the drill string and to provide a return pathway outside the drill string for drilling fluids. In producing operations, a plurality of production risers are used to provide a pathway for the transmission of hydrocarbons or other production fluid from multiple wells to the deck.
Each riser is typically projected up through an opening referred to as a moon pool in the vessel to working equipment and connections proximate an operational floor on the vessel. A riser pipe operating on the floating vessel in water depths greater than about 200 feet (34.72 meters) can buckle under the influence of its own weight and the weight of drilling fluid contained within the riser if it is not partially or completely supported. For floating platforms, the risers must be tensioned to maintain each riser within a range of safe operating tensions as the work deck moves relative to the upper portion of the riser. If a portion of the riser is permitted to go into compression, it could be damaged by buckling or by bending and colliding with adjacent risers. It is also necessary to ensure that the riser is not over-tensioned when the vessel hull moves to an extreme lateral or vertical position, such as, for example, when under extreme wave conditions or when ocean currents exert a significant side loading on the riser.
There are two primary types of tensioning systems: those that use long-stroke top-mounted tensioners (hydraulic, pneumatic, or hydra-pneumatic cylinders connected between the top of the riser and the vessel hull); and those that use buoyancy can tensioners (floatation devices connected to the upper portion of the each riser). The top-mounted tensioning systems function are either passive or active. The passive top-mounted tensioning systems include long-stroke tensioners that utilize cylinders with a stroke of typically between 15 and 30 feet in order to compensate for the movement expected due to deepwater operations. The active top-mounted tensioning systems further include a control system that actively adjusts hydraulic pressure of each long stroke tensioner cylinder to maintain a relatively constant tension on its associated riser. For both the passive and active top-mounted tensioning systems, abrupt lateral and vertical movements of the vessel hull are compensated for by the stroke of the tensioner. The buoyancy can tensioning systems, on the other hand, function by connecting buoyant cans, either individually, or collectively, to the top of each riser at a location below the water line to maintain a relatively constant tension, with abrupt lateral and vertical movements of the vessel hull being compensated for by allowing the buoyancy can and/or riser to slide up and down guide supports extending through the hull.
One of the problems related to offshore platforms that operate in deep and ultra-deep water (5000-10000+ foot water depth) is the amount or degree of lateral offset that is associated with the platform. The lateral offset, which results in a vertical differential between riser and vessel, is essentially controlled by the type of platform and the mooring system that is utilized. As water depths become deeper, however, regardless of the platform used, the lateral offsets increase. With floating production platforms such as SPAR's, which typically employ a top tensioned system of long-stroke tensioners, this lateral offset drives the total stroke requirements of the tensioning system. As a result, the stroke requirements can easily exceed 25-30 feet and, in active tensioning systems, require actively adjusting hydraulic pressure to increase pressure in the tensioning cylinders needed to maintain sufficient tension on the riser during a heave downward by the vessel and to reduce pressure in the tensioners to prevent the application of excessive tension to the risers during a heave upward by the vessel.
As such, it has been recognized by the inventors that the conditions associated with deep and ultradeep water inevitably result in a tensioning system made up of multiple cylinders, typically upwards of 25 feet in length, and capable of stroking the required 25-30 feet, along with significant space requirements within the vessel and/or an additional support frame or deck to support the non-stroking portion of the tensioner's cylinders, which can greatly add to the cost of the vessel to accommodate the 25-30 ft. stroke. By analogy, this can be equated to having to build a house with 25 foot high doorways and ceilings in every room of the house to accommodate the stroking portion of the tensioner's cylinders, rather than a normal single story having a six or eight foot doorway. Further, active tensioning systems can require a computerized control and feedback system and additional accumulators, gas pumps, pressure sensors, etc. These long-stroke tensioners can add significant extra weight to the hull supporting the production platform, and can significantly add to the costs of the riser management system. As exploration takes the industry into areas where the environmental and operational conditions, it is anticipated that there will be more and more instances where conditions exceed the current stroke capabilities of long-stroke top-mounted tensioning systems, which can lead to even higher costs.
Alternative designs to the long-stroke top-mounted tensioning systems have been employed to resolve the total stroke requirements. Such alternatives include the employment of a multi-buoyancy can system, described previously, which includes a set of individual buoyancy cans separately connected to a corresponding set of individual risers. Such systems, however, have some significant disadvantages. Such disadvantages include installation complexity, questionable storm resistance, stick-slip issues (e.g., due to contact with the side walls of the guide supports or columns), and buoyancy force limitations (e.g., resulting from a trade-off between the size of the individual buoyancy cans, the number of cans and risers supportable by the hull, and hull size. That is, for a given size hull, the larger the can the less number of risers supportable by the vessel. Similarly, for a given number of risers, the larger the cans, the larger the hull must be to support the risers, and the larger the costs of building, maintaining, and operating the vessel.
Another alternate platform design that solves some of these issues uses a “de-coupled” platform approach. Examples of such alternative platform design includes the riser support systems described, for example, in U.S. Pat. No. 7,537,416 and in U.S. Patent Publication No. 2009/009545, each incorporated by reference in its entirety. In essence, to employ such de-coupled approach, multiple production risers are immovably attached to one common, large air “mono-can” in such a fashion that the risers extend through the interstitial space between the air can cells, thus, de-coupling the production risers from the hull. In this design approach, the hull structure is laterally restrained and is independently moored and detached from the mono buoyancy can platform so as to allow the mono buoyancy can platform and risers to slide up and down guide supports extending through the hull. That is, this design approach employs the mono-can assembly as its substitute for long stroke tensioners to compensate for lateral offset.
Recognized by the inventors, however, is that while most of the total stroke requirements of a riser are directly related to vessel offset which generally effects each riser of a set of risers in a same manner and level, and thus, can be compensated for through utilization of a single buoyancy can platform, a small percentage of the stroke requirements are a result of factors which can affect each separate riser of the set of risers in a different manner or at least to a different level. These factors can include, for example, a change in riser initial length, riser initial weight, riser initial pre-tension, thermal growth, subsea wellhead and surface tree spacing distance, and pressure differentials between risers, which cannot be readily compensated for by a single de-coupled buoyancy can platform. Accordingly, it is recognized that although the single mono-buoyancy can- (multiple riser) decoupled platform system is an improvement upon the single-buoyancy can (single riser) platform approach, the mono-buoyancy can (decoupled) platform system still falls short of replacing long-stroke top-mounted tensioning systems, as it does not resolve this “small” but significant percentage of stroke requirements. It is further recognized that, as a result, such system will be expected to cause large tension variations between risers being held by the mono buoyancy can platform, as it assumes that environmental and operational variations have an equal effect on each riser in the set of risers, which can resultingly at least reduce the service life of one or more the risers, if not ultimately result in a catastrophic failure of one or more of the risers.
Accordingly, the inventors have further recognized the need for a riser tensioning system which can compensate for both lateral offset and additional factors such as thermal growth, subsea wellhead and surface tree spacing distance, and pressure differentials between risers, among others, without the need for long-stroke tensioners, or more significantly, the associated costs to the riser management system and the vessel associated with accommodating their significant size requirements.